A star is generally defined as an object large enough to have sustained nuclear fusion going on it is core. “Planet” doesn’t have a great definition, but they form around stars from a protoplanetary disk that contains material leftover from the star’s formation.

In between the two lie the brown dwarfs: intermediate objects that are massive enough to fuse lighter elements like deuterium (an isotope of hydrogen) and lithium, but quickly run out of fuel and become, for lack of a better word, inert. 

The range of brown dwarf masses is very roughly 13 to 77 times the mass of Jupiter — it depends on a lot of characteristics besides mass, but this is a decent rule of thumb.

But there’s more to it. Stars form from the direct collapse of gas from a gas cloud as it shrinks and forms a disk; this is called fragmentation. Think of it as top-down formation. Planets form as smaller bits of rockier materials in the disk clump up, growing bigger and bigger, colliding and merging, until a planet arises. This is called core accretion, and you can think of it as bottom-up formation.

Brown dwarfs, irritatingly, sit between these two mass regimes. Which way do they form? More massive ones, closer to stellar mass, probably form like stars, from fragmentation. But with ones closer to the lower limit the formation mechanism isn’t clear.

To look into it, astronomers observed 29 Cygni b, an object with a mass of about 15 ± 5 times that of Jupiter, putting it right on the “deuterium limit”, the lowest mass object that can fuse deuterium. So it may be a very low-mass brown dwarf or a very high-mass planet. The host star is an A-type (about twice the mass of the sun) roughly 133 light-years from us [link to journal paper]. 

On top of that, 29 Cygni b orbits the star pretty far out, about 2.4 billion kilometers away, a bit closer in than Uranus orbits the sun. That far from the star the protoplanetary disk isn’t very dense, so it’s hard for planets to form from accretion. In that case they likely either form farther in and get gravitationally tossed out by a close encounter with another planet, or they form from fragmentation.

When it all clicks.

Why does business news feel like it's written for people who already get it?

Morning Brew changes that.

It's a free newsletter that breaks down what's going on in business, finance, and tech — clearly, quickly, and with enough personality to keep things interesting. The result? You don't just skim headlines. You actually understand what's going on.

Try it yourself and join over 4 million professionals reading daily.

Check it out

One thing that separates the two processes is the amount of heavier elements the planet draws in. Fragmentation tends to generate lower amounts of these elements (like silicon, carbon, and oxygen), whereas accretion by definition gets more of them (since you need rockier materials to glom together; also, this tends to draw in more material with heavy elements in it later on due to some complicated physics).

The astronomers used JWST to take a peek at 29 Cygni b. It’s far enough out from the star that they could get direct images of it by using an occulting mask (basically a disk or bar of metal in the pathway light takes inside the telescope) to block the star’s glare, allowing the fainter companion to be seen. They used filters that pick out the infrared light emitted by carbon dioxide and carbon monoxide molecules, because that can be used to find the ratio those two molecules in the companion’s atmosphere, and from there how abundant heavier elements are.

What they found is that the ratio does lean toward the companion forming bottom-up, through accretion, so more like a planet than a star.

They also did a clever follow-up, using an interferometer to look at the star itself. This is a technique that allows extremely high-resolution images on very tiny areas of the sky, so hi-res that the shape of a star can be detected! A-type stars are fast rotators, and tend to be oblate, flatted, due to centrifugal force. They measured that for the star, and found the planet orbits in more or less the same plane as the star’s equator. Protoplanetary disks tend to form in the equatorial plane, so again that implies the companion formed like a planet, not a star. This isn’t a smoking gun, but it does lend credence to the accretion mechanism.

 That’s pretty interesting! If this holds up it shows that objects can form like planets even past the deuterium limit, well into the brown dwarf regime (assuming the mass of 29 Cygni b really is 15 Jupiters; the uncertainty of 5 Jupiter masses up or down is a bit large for my taste to be sure).

Become a paying subscriber of Premium to get access to this post and other subscriber-only content.

Already a paying subscriber? Sign In.

A subscription gets you:

• • Three (3!) issues per week, not just one
• • Full access to the BAN archives
• • Leave comment on articles (ask questions, talk to other subscribers, etc.)